MCAT Biology Question — Aldosterone Action

increasing water permeability of the ascending limb of the loop of Henle

increasing water permeability in the proximal tubule

increasing sodium absorption and potassium secretion in the distal tubule

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Filtrate pressure in the distal tubule is monitored by the juxtaglomerular apparatus. Special cells in the juxtaglomerular apparatus secrete an enzyme called renin either when stretch receptors in vessel walls detect a blood pressure drop (decreased renal perfusion) or when macula densa cells in the distal convoluted tubule detect a drop of sodium concentration in the tubular filtrate. Renin release triggers a cascade of angiotensin I, II, and III which in turn causes the adrenal cortex to secrete aldosterone. Aldosterone acts on the collecting tubule of the distal tubule (not the collecting duct) to increase membrane proteins absorbing sodium and secreting potassium. For the MCAT: decreased [Na+]filtrate or decreased blood pressure -> juxtaglomerular apparatus releases renin -> angiotensin I,II,III cascade -> aldosterone -> increased distal tubule Na+ absorption from and K+ secretion into the filtrate. Increased blood pressure is a secondary effect of the aldosterone.

A. Increasing water permeability of the collecting duct, incorrect, Antidiuretic Hormone (also known as ADH and vasopressin) causes increased water permeability in the collecting duct.

B. Increasing water permeability of the ascending limb of the loop of Henle, incorrect, The ascending limb of the loop of Henle is nearly impermeable to water.

C. Increasing water permeability in the proximal tubule, incorrect, Water reabsorption occurs in the descending limb of the loop of Henle, the collecting tubule, and the collecting duct.